Goto the video, right-click on it and choose to save the video to your dekstop. Then, ctrl + o, browse to the mp4 file and open it.

Does it play then? If it does, then we at least know your Opera is capable of parsing the file and playing its parts. That would mean then that it's some issue with parsing and playing the bits on the fly while it's streaming. Could be a firewall and or antivirus issue then. Or, it could be a proxy issue.